give free rein to (one)
To give one complete freedom to do what one wants or chooses. "Rein" refers to the reins of a horse. Can you believe the boss gave free rein to me? Finally, I can present a campaign with my own vision!
give free rein to someone
and give someone free reinFig. to allow someone to be completely in charge (of something). (Alludes to loosening the reins of a horse and therefore control.) The boss gave the manager free rein with the new project. The principal gave free rein to Mrs. Brown in her classes.
give free rein to
see under free hand.